This study will examine the role of the human placenta in the regulation of steroid hormone biosynthesis during pregnancy. It will analyze the manner and extent to which the general metabolic functions of the placenta coordinate with its endocrine functions and thereby, promote, limit or otherwise modulate estrogen and progesterone production, either selectively or synchronously. To detect and evaluate such metabolic control, the following placental activities have been selected for primary scrutiny: a) the generation of the reduced forms of the pyridine nucleotides; b) the utilization of these reduced cofactors for estrogen biosynthesis by the oxidases which catalyze aromatization; c) the utilization of these nucleotides for progesterone biosynthesis by the oxidase system which cleaves the side-chain of cholesterol; d) the generation of ATP and its participation in these other processes. This study will use a synthetic approach, examining the biochemical capacities and properties of isolated cell fractions of the human placenta and the interactions obtained upon their recombination and reconstitution into systems of greater and greater functional and structural complexity, so that finally the consequences of the complete integration of metabolic and endocrine activities in the intact cell can be studied, interpreted and understood.